Food processors using plastic containers for packaging foodstuffs such as cottage cheese, butter, etc. have traditionally used containers and lids made from thermoformed plastic materials. Thermoformed plastic packaging materials have been relatively inexpensive to packagers in terms of both low purchase prices and their light weight which minimized shipping costs. Thermoforming procedures have been performed using thin structurally strong plastic sheets which are formed at high speed over a large number of dies to simultaneously produce container components at high production rates.
Injection molded plastic packaging has been available but has not been a cost effective alternative to thermoformed elements. Recent advances in injection molding technology have made packaging produced this way economically competitive with thermoformed packaging. In particular, it has become possible to injection mold containers in multicavity molds at production rates which are highly competitive with the thermoformed products To enable the high production rates it is essential that the product design facilitate high injection flow rates simultaneously into multiple mold cavities e.g. "shooting" the plastic into a sixteen cavity mold in less than one second.
Because the improved technology has made injection molded packages relatively inexpensive, processors have begun to specify these containers and lids. A prerequisite of these containers is that they must be designed so that they can be accepted by existing packaging machinery which, in many cases, has been specifically constructed for handling thermoformed containers.
Plastic container forming materials lending themselves to injection molding processes tend to be relatively pliant, or easily flexed Great structural strength and rigidity is thus not a prime attribute of these injection molded containers. Accordingly such containers and lids must employ relatively heavy wall thicknesses where strength and rigidity are required At the same time the containers must be as light as possible to minimize both shipping and material costs.
The requirement for interchangeability with existing container manufacturing and packaging machinery is particularly critical. In the case of containers manufactured for packaging retail consumer products (e.g. dairy products) the containers are typically printed with labeling and brand information as they are being manufactured Printing requires container surfaces which readily accept printed indicia. Further, the container walls must coact with existing container printing equipment so that high quality images can be consistently transferred to the containers. If the container wall is deflected away from the indicia printing member at the time when an image is to be transferred the printed image is discontinuous or of varying density. Prior art containers employing variable thickness sidewalls have experienced image problems of this kind which result in unsightly packages.
After filling the container with such a product it is hermetically closed by a removable lid. This operation takes place in capping machinery. The capping machinery forces each lid onto a container and in so doing subjects the container to crushing forces These forces tend to collapse and buckle the container side wall inwardly. This action, while not usually sufficient to hole the side wall, tends to spew the contents into the machinery and/or to prevent establishing an effective seal between the lid and the container.
Because the containers are not extremely tall and the contents are not maintained under superatmospheric pressure the maximum bursting pressure exerted on the sidewall is slight. The container side wall thickness need only be minimal to resist bursting forces, yet the side wall must have "column" strength to resist the capping forces.
The disparate requirements of the injection molded containers have tended to result in containers which are heavier and more expensive than actually required for packaging.
The present invention provides a new and improved injection molded plastic container which is produced efficiently and inexpensively, uses minimal material so that its weight and material cost are minimized yet which provides relatively great column strength to resist crushing and permit efficient image transfers during printing.